In an optical time division multiplexed (TDM) transmission system a number of channels, hereinafter referred to as tributary channels, at a relatively lower bit rate are multiplexed to provide a single multiplexed (aggregate) channel at a relatively higher bit rate. For example there may be tributary channels of 10 Gbit/s, with perhaps 2 to 8 tributaries, to make an aggregate multiplexed signal of up to 80 Gbit/s or more. In a TDM system multiplexing N tributary channels, the channels are usually combined by allocating time slots 1 to N so that every Mth bit comes from the Mth tributary channel. At the far end of a transmission link the incoming signal needs to be demultiplexed into its original N tributary channels for onward distribution as required. Clearly the multiplexed bits arrive at the far end of the link in the same order as they were originally multiplexed, but conventionally at this far end there is no information to identify which bit belongs to which tributary channel except from data information carried within the data streams of those tributary channels.